1. Field of the Invention
The present invention relates to the field of digital communications, and more specifically, is directed to a system for acquiring or recovering a symbol timing offset and carrier frequency error from a digitally modulated, multi-carrier communications signal. The system of the present invention is further directed to a diversity system for recovering a symbol timing offset and carrier frequency error from a diverse, multi-frequency digitally modulated signal.
2. Prior Art
In a digital communication system, the acquisition of symbol synchronization between the transmitter and receiver must be achieved before effective symbol demodulation can commence at the receiver. Thus, the prior art in digital communications is replete with attempts to develop systems directed to the optimal recovery or acquisition of received symbol timing and carrier frequency.
One such attempt is provided in U.S. Pat. No. 5,541,552, by Suzuki, directed to the demodulation of digitally modulated multi-carrier symbols. The receiver in this Patent develops correlation peaks responsive to an intra-symbol correlation, as by conjugate product formations between the repetitive information and the amplitude tapering existing at the leading and lagging portions of each received symbol. The peaks must be detected, for their positions are indicative of received symbol timing. However, their relatively flat amplitude profiles necessitate their individual integration prior to the peak detection thereof.
This simplistic signal processing scheme in Suzuki, i.e., mere integration of correlation peaks prior to their detection, provides sub-optimal synchronization results on at least two accounts. First, as the number of frequency carriers is increased in the multi-carrier symbol, the correlation peak becomes obscured by adjacent noise, and mere integration fails to sufficiently enhance the signal-to-noise ratio of the pre-detected correlation peaks. Second, mere integration fails to combat well known signal propagation effects, such as scattering, correlation peaks. Second, mere integration fails to combat well known signal propagation effects, such as scattering, fading, or other signal interferers, all of which combine to cause signal drop-outs and false peaks.
In contrast to Suzuki and other prior art acquisition systems, the system of the present invention presents an optimum scheme for the recovery of symbol timing and carrier frequency. This optimum approach is provided through an application of signal processing techniques that, when taken either alone or in combination, improve over the prior art. These signal processing improvements include the additive superposition of repeating signals, optimum or matched filtering, and diversity selection and combining between diverse, modulated signals.
The following is a list of prior art references known to Applicant: U.S. Pat. Nos. 5,694,389; 5,602,835; 5,608,764; 5,559,833; 5,687,165; 5,541,552; 5,357,502; 3,925,650; 5,596,582; 3,364,482; 2,943,316; 3,975,687; 5,594,761; 4,281,412; 5,187,711; 4,727,534; 5,369,800; 5,548,819; 2,549,423; 2,880,275; 3,555,427; 5,629,639; 5,428,647; 5,682,376; 5,416,767; 5,452,331; 4,344,180; 5,657,313; 5,652,772; 5,627,863; 5,550,812; 5,506,836; 5,471,464; 5,450,456; 5,371,761; 5,345,440; 5,313,169; 5,228,025; 5,191,576; 5,371,548; 5,406,551; and, 3,780,279.
It is an object of the present invention to provide a system for optimally recovering symbol timing and carrier frequency from a digitally modulated, multi-carrier signal. Particularly, the digitally modulated signal is an orthogonal frequency division multiplexed (OFDM) signal that can include a large number of frequency carriers therein.
One aspect of the present invention presents a system for recovering a symbol timing offset and carrier frequency error from an OFDM signal. The system includes a receiver module for receiving the OFDM modulated signal which is represented as a series of OFDM symbols. Each OFDM symbol includes a leading portion and a trailing portion thereof. The leading and trailing portions have substantially equivalent phases and have temporally weighted amplitudes according to a predetermined temporal weighting function. The receiver module provides a received signal to an output thereof.
The system also includes a signal peak development module which develops a boundary signal having a plurality of signal peaks representing a symbol boundary position for each of the received OFDM symbols. Each of the signal peaks is developed responsive to an amplitude and phase correspondence produced between the leading and trailing portions of the received OFDM symbol.
The system further includes a signal enhancing module that enhances signal peak detectability of the boundary signal. The signal enhancing module includes a circuit for additively superimposing a plurality of segments of the boundary signal. The signal enhancing module outputs an enhanced signal peak corresponding to a predetermined number of the plurality of segments and having an. improved signal-to-noise ratio.
The system includes a circuit for establishing a temporal position indicative of the symbol boundary position from at least one of the enhanced signal peaks output from the signal enhancing module. The temporal position represents a received OFDM symbol timing offset.
A circuit for recovering the received OFDM signal carrier frequency error in correspondence with said temporal position is also included in the system.
In another aspect of the present invention, the signal enhancing module further includes a matched filter for filtering signals produced responsive to the boundary signal. The matched filter further improves a signal-to-noise ratio of the enhanced signal peaks output from the signal enhancing module. The matched filter has a temporal impulse response matched to the amplitude envelope of the signal peaks present in the boundary signal.
Another object of the present invention is the improvement of the symbol timing and frequency carrier acquisition process through the application of signal diversity selection and combining. In the diversity aspect of the present invention, a diversity system for recovering a symbol timing offset and carrier frequency error from the OFDM signal includes a transmitter for transmitting an OFDM modulated signal in both an upper and a lower frequency sideband. Each of the sidebands encompasses a plurality of OFDM modulated frequency carriers and a series of OFDM symbols.
The diversity system includes a receiver portion for receiving the OFDM modulated signal of the upper frequency sideband and providing a first or upper sideband received signal output and the OFDM modulated signal of the lower frequency sideband and providing a second or lower sideband received signal output.
The diversity system also includes a first recovery circuit for developing a first or upper sideband symbol timing offset corresponding to OFDM symbols in the first received signal, and a second recovery circuit for developing a second or lower sideband symbol timing offset corresponding to OFDM symbols in the second received signal.
Further, the diversity system includes a first validation circuit for validating the first symbol timing offset based on a predetermined temporal consistency thereof, and a second validating circuit validating said second symbol timing offset based on a predetermined temporal consistency thereof.
Still further, the diversity system includes a selection circuit for selecting an optimum symbol timing offset from the first and second symbol timing offsets responsive to signals output from the first and second validating circuits, and a demodulator circuit for demodulating at least one of the series of OFDM symbols using the optimum symbol timing offset.
A further aspect of the diversity system includes a circuit for developing a third or combined symbol timing offset corresponding to a combination of first and second timing signals respectively representative of the first and second symbol timing offsets, and a circuit for selecting the third symbol timing offset responsive to a plurality of conditional input signals.